The present invention relates to masking tape adhered to the rear surface of a lead frame to protect that rear surface while a die attached to the front surface of the lead frame is embedded in resin. In particular, the invention relates to methods and apparatus for removal of the masking tape.
Following attachment of a die (that is an integrated circuit package) to a front surface of a lead frame, it is conventional to encase the die in resin by a moulding process. This is carried out by placing the lead frame at the bottom of a mould, with the front surface of the lead frame facing upwardly. Liquid resin is inserted into the mould so that the die is submerged in it. Gradually the resin hardens so as to encase the die. It is highly preferable to prevent the resin from contaminating the rear surface of the lead frame (i.e. the surface facing away from the die), particularly because, when the die is subsequently in use, resin on the rear surface may inhibit the escape through that surface of heat generated within the die.
For this reason, it is known to adhere masking tape to the rear surface of the lead frame during the moulding process. After the moulding is complete, the lead frame is inverted so that the rear surface is uppermost, the tape is removed from the rear surface by tearing it away, and a vacuum source is used to suck the discarded tape away for disposal. Once the tape has been removed, further process steps, such as post-mould curing may be conveniently carried out.
The present inventors have now appreciated that this known process involves a number of disadvantages. Firstly, the tearing process may subject the die to sufficiently severe mechanical stresses to risk damage to the die. Secondly, the process of turning the lead frame over complicates the process, and thus increases its cost. Thirdly, since the discarded tapes still carry adhesive they retain a stickiness, and can xe2x80x9cgum upxe2x80x9d the operation of the vacuum source.
The present invention seeks to provide new and useful methods and apparatus for removing the masking tape from the rear surface of a lead frame following the moulding of resin to the front surface of the lead frame.
In a first aspect, the invention proposes that while the masking tape is peeled away, the lead frame is clamped by a clamping surface which presses on the front surface of the lead frame. To reduce the risk of damage to the die and lead frame (and the connections between them), the clamping surfaces either includes recess(es) for receiving the moulded resin body (or bodies), or is deformable to create such recesses. Thus, clamping force is transmitted to the lead frame other than entirely through the resin bodies.
Specifically, a first aspect of the invention is a method of removing a section of masking tape adhered to a rear surface of a lead frame, at least one die being provided on a front surface of the lead frame opposite the rear face, the die being encased in a moulded resin body,
the method comprising the steps of:
supporting the lead frame on its rear surface;
urging a clamping surface of a clamp plate towards the front surface of the lead frame to clamp the lead frame, the clamping surface receiving the or each resin body within at least one recess in the clamping surface; and
peeling the masking tape from the rear surface of the lead frame.
Note that clamping the lead frame makes it possible to peel the masking tape away from the lead frame while the rear surface of the lead frame is still facing downwardly. In other words, it is unnecessary according to the invention to invert the lead frame after the moulding and before the peeling step. This significantly reduces the complexity, and hence the cost, of the process step.
Furthermore, the clamping method ensures that clamping force is distributed widely over the front surface of the lead frame, much reducing the chance that damage will be caused to the lead frame during the peeling process by a concentration of force.
A second aspect of the invention proposes in general terms that the peeling itself is conducted in a two-stage automatic process. In a first stage an edge of the masking tape is separated from the lead frame by a first motion which is does not stretch it (e.g. a substantially diagonal motion). In a second stage the masking tape is peeled back from the lead frame by a second motion parallel to the rear surface of the frame.
Specifically, in the second aspect the invention proposes a method of removing a section of masking tape adhered to the rear surface of a lead frame and extending in a first direction parallel to the rear surface of the lead frame, the method comprising:
attaching a gripping device to an edge portion of the tape which is furthermost in the first direction;
moving the gripping device relative to the lead frame by a predetermined distance in a direction having a first component opposite to the first direction and a second component away from the surface, whereby the edge portion of the tape is moved away from the lead frame by a predefined distance; and
moving the gripping device relative to the lead frame oppositely to the first direction, whereby the masking tape is gradually peeled from the rear surface of the lead frame.
A third aspect of the invention relates in general terms to the disposal of the masking tape after it is peeled from the lead frame. Instead of sucking it away with a vacuum source, the third aspect of the invention proposes that the surface of the masking tape which was previously adhered to the lead frame (and which will still generally be covered with adhesive) is adhered to a retrieval surface pressed against it. Thus, the second surface of the masking tape (i.e. the one which was not adhered to the lead frame) would then face away from the retrieval surface.
Subsequently, when the masking tape of the next lead frame has been peeled away, its side which is covered with adhesive is pressed against the second surface of the first masking tape, and adhered to it. In this way a stack of masking tapes can be built up. All the masking tapes can then be disposed of together. No vacuum source is required, and in particular none is required while the dies and lead frames are still being processed.
Specifically, a first expression of the third aspect of the invention is a method of removing a section of masking tape from a lead frame, the masking tape having a first surface adhered to a rear surface of the lead frame and extending in a first direction parallel to the rear surface of the lead frame, the method comprising:
gripping an edge portion of the masking tape which is furthermost in the first direction,
peeling the masking tape from the lead frame by moving the edge portion opposite to the first direction, whereby the first surface of the masking tape is turned to face away from the lead frame; and
urging a retrieval surface of a retrieval plate against the first surface of the masking tape, whereby the masking tape becomes adhered to the retrieval surface.
Taking into account the possibility of stacking multiple tapes, an alternative expression of the third aspect of the invention is a method of removing respective sections of masking tape from respective rear faces of a plurality of lead frames, each section of masking tape having a first surface adhered to the rear surface of the respective lead frame and extending in a first direction parallel to the rear surface of the lead frame, the method comprising:
(i) gripping an edge portion of the masking tape of the first lead frame, the edge portion being furthermost in the first direction,
(ii) peeling the masking tape from the first lead frame by moving the edge portion oppositely to the first direction, whereby the first surface of the masking tape is turned to face away from the lead frame;
(iii) urging a retrieval surface of a retrieval plate towards the first surface of the masking tape, whereby the masking tape of the first lead frame becomes adhered to the retrieving surface; and
(iv) repeating steps (i) to (iii) successively for the other lead frames, in each step (iii) the masking tape of the respective lead frame becoming adhered to the masking tape of the previous lead frame, whereby the masking tapes of the plurality of lead frames are successively stacked on the retrieval surface.
Note that the three aspects of the invention are freely combinable. For example, the peeling and disposal concepts which are the second and third aspects of the invention are preferably carried out while the lead frame is clamped according to the first aspect of the invention. Similarly, the disposal which is the third aspect of the invention is carried out after a peeling step which is preferably, but not necessarily, according to the second aspect of the invention.
Although the invention has been described above in terms of methods, it can equally well be expressed in term of apparatus for automatically carrying out the various methods.